Low haze UV blocking removable lens stack

ABSTRACT

A protective lens stack comprises a base layer including a polyethylene terephthalate (PET) film containing a UV blocking additive and one or more removable lens layers stacked on top of the base layer. Each of the removable lens layers may include a PET film and a first adhesive disposed on a bottom surface of the PET film. Refractive indices of the base layer and the one or more removable lens layers may be matched to within 0.2. Each layer of the protective lens stack, including the base layer and each of the removable lens layer(s), may have less than 0.5% haze. The protective lens stack may be used as a face shield or may be surface mountable using a second adhesive disposed on a bottom surface of the PET film containing the UV blocking additive.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND

In an operating room environment, there is a need for a protective faceshield with low haze, removable outer layer(s), ultraviolet (UV)radiation blocking properties, and stabilization during thesterilization process, both gamma irradiation and ethylene oxide (EO)sterilization.

Haze is an optical phenomenon which happens in both reflection andtransmission. In reflection, the eye sees glare, scatter, and halos. Intransmission, the visible light is reduced or darkened by internalscatter and is seen by the eye as milky and not clear. In order to meetthe American National Standard for Occupational and Educational PersonalEye and Face Protection Devices (ANSI Z87.1-2020) and be classified as aprotective face shield, the final product must have less than 3% haze.More than 3% haze in a face shield can give the user a dizzy or sickfeeling. Ideally, a protective face shield used in the operating roomshould have 1% or less haze.

Removable outer layer(s) are needed when the surgeon or operatingpersonnel get splatter that impairs their view during a procedure.Cleaning or wiping the face shield leads to smearing, and breaking thesterile field in the operating room to change face shields and gowns istime consuming and adds to the risk of infecting the patient.

UV blocking is necessary in the operating room to protect the operatingpersonnel's face and eyes because some hospitals use lighting which cangenerate UV rays: quartz, halogen, tungsten or xenon. Some hospitalsalso use UVC lights to help reduce airborne pathogens.

Stability during sterilization is critical for both EO and gammairradiation sterilization processes. Single-use medical products usedduring surgery must be sterile. Common methods to sterilize plasticsinclude gamma irradiation and EO gas. After either or both processes,the laminated stack must be free from viable micro-organisms (includingin between each laminated layer). Additionally, the optical stackperformance and physical characteristics cannot change, including butnot limited to discoloration, outgassing, optical defects, ordelamination.

The current state-of-the-art solution is a laminated stack ofindex-matched removable lenses made from polyester film (PET). Opticalquality PET is super clear with low haze and can be sterilized with EOand gamma without negatively impacting the optics, tensile strength orcolor. Polycarbonate, for example, can turn brown in color under gammasterilization. However, PET is transparent to ultraviolet radiation(i.e. it transmits UV rays to the wearer). Therefore, a thin UV blockinglayer is added to the base layer PET. A common UV blocking film isHostaphan® 7333UV, made by Mitsubishi Polyester Film Group. The mostcommon gauge or thickness used for this application is 1 mil becausethis is the gauge with the least amount of haze. The 1-mil thick UVblocking PET film from Mitsubishi has 1% haze. A 7-mil thick UV blockingPET film has 4% haze. An example face shield construction beginning fromthe base layer may be 7-mil PET, permanent adhesive, 1-mil UV blockingPET, removable adhesive, 2-mil PET, removable adhesive, 2-mil PET.

The most common test method for haze and visual light transmission isthe Standard Test Method for Haze and Luminous Transmittance ofTransparent Plastics (ASTM D1003). It utilizes a haze meter andspectrophotometer. The above-described state-of-the-art protective faceshield results in haze of 2.05% and VLT of 88.7% according to thisstandard. This meets the specification for a protective face shield(ANSI Z87.1-2020) but has twice the haze of an ideal operating room faceshield. Considering that haze in an optical stack is additive, it ispreferred that each layer in the stack has less than 0.5% haze.

BRIEF SUMMARY

The present disclosure contemplates various protective lens stacks andface shields for overcoming the above drawbacks accompanying the relatedart. One aspect of the embodiments of the disclosure is a protectivelens stack. The protective lens stack may comprise a base layerincluding a polyethylene terephthalate (PET) film containing a UVblocking additive. The protective lens stack may further include one ormore removable lens layers stacked on top of the base layer, each of theremovable lens layers including a PET film and a first adhesive disposedon a bottom surface of the PET film. Refractive indices of the baselayer and the one or more removable lens layers may be matched to within0.2.

370 nm transmission of the protective lens stack may be less than 5%.Transmission of the protective lens stack at wavelengths of 320-380 nmmay be less than 5%.

The UV blocking additive may comprise a hindered amine light stabilizer(HALS).

The UV blocking additive may comprise a UV absorber. The UV absorber maybe selected from the group consisting of carbon black, rutile titaniumoxide, hydroxy benzophenone, and benzophenone.

The UV blocking additive may comprise a quencher.

The PET film of the base layer may have a thickness of 4 mil to 20 mil,preferably 7 to 8 mil (e.g. 7.5 mil).

The PET film of each of the one or more removable lens layers may have athickness of 1 to 4 mil, preferably 2 mil.

The base layer may include a second adhesive disposed on a bottomsurface of the PET film containing the UV blocking additive, the secondadhesive having a pull strength greater than that of the first adhesive.The PET film of the base layer may have a thickness of 2 mil to 7 mil,preferably 3 to 4 mil. The PET film of each of the one or more removablelens layers may have a thickness of 2 to 7 mil, preferably 3 to 4 mil.

A total haze of the protective lens stack may be 2% or less (e.g. 1%). Avisible light transmission (VLT) of the protective lens stack may be 87%or greater (e.g. 90%).

Another aspect of the embodiments of the present disclosure is a faceshield. The face shield may comprise a base layer including apolyethylene terephthalate (PET) film having a thickness of 4 mil to 20mil and containing a UV blocking additive. The face shield may furthercomprise one or more removable lens layers stacked on top of the baselayer, each of the removable lens layers including a PET film having athickness of 1 to 4 mil and an adhesive disposed on a bottom surface ofthe PET film. Refractive indices of the base layer and the one or moreremovable lens layers may be matched to within 0.2.

Another aspect of the embodiments of the present disclosure is a surfacemountable protective lens stack. The surface mountable protective lensstack may comprise a base layer including a polyethylene terephthalate(PET) film having a thickness of 2 mil to 7 mil and containing a UVblocking additive. The surface mountable protective lens stack mayfurther comprise one or more removable lens layers stacked on top of thebase layer, each of the removable lens layers including a PET filmhaving a thickness of 2 to 7 mil and a first adhesive disposed on abottom surface of the PET film. The base layer may further include asecond adhesive disposed on a bottom surface of the PET film containingthe UV blocking additive, the second adhesive having a pull strengthgreater than that of the first adhesive. Refractive indices of the baselayer and the one or more removable lens layers may be matched to within0.2.

Another aspect of the embodiments of the present disclosure is aprotective lens stack. The protective lens stack may comprise a baselayer including a polyethylene terephthalate (PET) film containing a UVblocking additive, the base layer having less than 0.5% haze. Theprotective lens stack may further comprise one or more removable lenslayers stacked on top of the base layer, each of the removable lenslayers having less than 0.5% haze and including a PET film and a firstadhesive disposed on a bottom surface of the PET film.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the various embodimentsdisclosed herein will be better understood with respect to the followingdescription and drawings, in which like numbers refer to like partsthroughout, and in which:

FIG. 1 is a cross-sectional view of a protective lens stack according toan embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure encompasses various embodiments of an opticalstack of laminated removable protective lenses with a low hazeintegrated UV absorbing packaging for using as a protective face shieldor affixing to a window (e.g. of a surgical helmet, hood, or gown wornin an operating room). The detailed description set forth below inconnection with the appended drawings is intended as a description ofseveral currently contemplated embodiments and is not intended torepresent the only form in which the disclosed invention may bedeveloped or utilized. The description sets forth the functions andfeatures in connection with the illustrated embodiments. It is to beunderstood, however, that the same or equivalent functions may beaccomplished by different embodiments that are also intended to beencompassed within the scope of the present disclosure. It is furtherunderstood that relational terms such as first and second and the likeare used solely to distinguish one from another entity withoutnecessarily requiring or implying any actual such relationship in orderbetween such entities.

FIG. 1 is a cross-sectional view of a protective lens stack 100according to an embodiment of the present disclosure. The protectivelens stack 100 may be affixed to a surface 10 as shown or may standalone as a face shield, which may be attached at a perimeter thereof toa frame, for example. The surface 10 may be a transparent window of asurgical helmet, hood, or gown as may be worn by a surgical team memberin an operating room. In the case of a protective lens stack 100 used asa standalone face shield without being mounted to a surface 10, the faceshield may be of an open type that is worn in front of and onlypartially encloses the face. Alternatively, the face shield may be of aclosed type such as a visor of a helmet or other article that fullyencloses the wearer's face and head.

In either case, the protective lens stack 100 may combine low haze, UVblocking, and stability under multiple sterilization processes, allwhile providing one or more removable lens layers that may be torn offby the wearer as they become soiled. To this end, the protective lensstack 100 may comprise a base layer 110 including a polyethyleneterephthalate (PET) film 112 containing a UV blocking additive and oneor more removable lens layers 120 stacked on top of the base layer 110.By incorporating the UV blocking additive into the PET film 112 of thebase layer 110, the desired UV blocking capability can be achievedwithout the need to include a separate UV blocking film or permanentadhesive layer therebetween. By eliminating these two layers, the totalhaze of the protective lens stack 100 may be reduced by 1%, amounting toa 50% increase in clarity over the conventional construction. Each layerof the resulting protective lens stack 100, including the base layer 110and each of the removable lens layer(s) 120, may have less than 0.5%haze.

Each of the removable lens layers 120 may include a PET film 122 and anadhesive 124 disposed on a bottom surface of the PET film 122. The PETfilm 122 of the removable lens layer(s) 120 need not include the UVblocking additive since the removable lens layer(s) 120 are intended assacrificial layers that can be removed as they become soiled. Theadhesive 124 may be a dry mount adhesive such as a polymethylmethacrylate (PMMA) adhesive as described in U.S. Pat. No. 9,295,297,entitled “Adhesive Mountable Stack of Removable Layers,” a wet mountadhesive as described in U.S. Pat. No. 9,128,545, entitled “Touch ScreenShield,” or an adhesive as described in U.S. Pat. No. 6,536,045,entitled “Tear-off Optical Stack Having Peripheral Seal Mount,” theentire contents of each of which is expressly incorporated by referenceherein. The adhesive 124 may be a pressure sensitive adhesive (PSA).

In the example of FIG. 1, two removable lens layers 120 are shown: afirst removable lens layer 120 stacked on top of the base layer 110 anda second removable lens layer 120 stacked on top of the first removablelens layer 120. The adhesive 124 of the first removable lens layer 120adheres the PET film 122 of the first removable lens layer 120 to thePET film 112 of the base layer 110. The adhesive 124 of the secondremovable lens layer 120 adheres the PET film 122 of the secondremovable lens layer 120 to the PET film 122 of the first removable lenslayer 120. In this way, a stack of removable lens layers 120 may beassembled, each one after the first being adhered to the PET film 122 ofthe preceding removable lens layer 120.

When the outermost PET film 122 becomes soiled (e.g. with blood in anoperating room setting), the wearer of the protective lens stack 100 maysimply tear off the outermost removable lens layer 120 to reveal a clearlens layer 120 or the base layer 110 underneath. The adhesive 124 ofeach removable lens layer 120 may remain with the PET film 122 of thesame removable lens layer 120 as the layer 120 is removed. That is, theadhesive 124 may be removable from the PET film 122 of the precedinglens layer 120 without leaving a significant amount of residue (e.g.with adhesive transfer 1% or less). So that it is easier to tear off theremovable lens layer(s) 120, each removable PET film 122 may have anadhesive-free region 123 or “dry lane” at a periphery thereof where theadhesive 124 is not applied. For example, in the case of a 10-inch-wideprotective lens stack 100 (having one or more 10-inch-wide removable PETfilms 122), the adhesive-free region 123 of each removable PET film 122may be less than one-third of the width of the PET film 122, e.g., 3.2inches. In the illustrated example, the adhesive-free region 123 is ononly one side of the PET film 122, but it is contemplated that theadhesive-free region 123 may be provided on both sides depending on thepreferred technique for tearing off the removable lens layer(s) 120(e.g. with left hand only, with right hand only, or with either hand).

In order to ensure high visual acuity through the protective lens stack100, internal reflections within the protective lens stack 100 may beminimized by matching the indices of refraction of each layer 110, 120.In this regard, the PET films 112, 122 and adhesive layers 124 making upthe layers 110, 120 may be produced or selected so as to have the sameor very similar indices of refraction as described in U.S. Pat. No.9,295,297, incorporated by reference above. In particular, refractiveindices of the base layer 110 and the one or more removable lens layers120 may be matched to within 0.2.

The UV blocking additive that is incorporated into the PET film 112 ofthe base layer 110 may take a variety of forms. Most preferably, the UVblocking additive may comprise a hindered amine light stabilizer (HALS),which is a long-term thermal stabilizer that traps free radicals formedduring photo-oxidation of plastics. The HALS may be optically clear andmay be produced or selected to be refractive index matched to the PETfilm 112 or, in general, such that the refractive indices of the baselayer 110 containing the HALS and the one or more removable lens layers120 are within 0.2.

Other examples of the UV blocking additive that may be incorporated intothe PET film 112 of the base layer 110 instead of or in combination withHALS are UV absorbers and quenchers. UV absorbers may include carbonblack, rutile titanium oxide, hydroxy benzophenone, and benzophenones,for example. However, carbon black may have the negative effect ofadding haze, while the other UV absorbers are either not optimized forthin PET films or insufficiently block UV radiation (e.g. rutiletitanium oxide may not block UVB below 315 nm). Quenchers containingheavy metals may provide limited UV blocking capability to the PET film112 (not as effective as HALS), but have the negative property of addingunwanted color to the resulting protective lens stack 100. In any case,the refractive index of the UV blocking additive may be matched withthat of the PET film 112 and the remainder of the protective lens stack100 (e.g. to within 0.2).

Owing to the incorporation of the UV blocking additive (HALS, UVabsorber, and/or quencher) into the PET film 112 of the base layer 110,UV rays can be blocked by the protective lens stack 100. In particular,the resulting 370 nm transmission of the protective lens stack may beless than 5%. More preferably, the resulting transmission of theprotective lens stack 100 at wavelengths of 320-380 nm may be less than5%. Moreover, since the UV blocking additive is polymer-bound in the PETfilm 112, it cannot migrate from the PET film 112 during sterilizationprocesses such as gamma irradiation and ethylene oxide (EO)sterilization. Therefore, the protective lens stack 100 may be used inan operating room setting and undergo the typical sterilizationprocesses without degradation.

The characteristics of the PET film 112 containing the UV blockingadditive may be within the ranges shown in Table 1, below.

TABLE 1 Characteristic Unit Range Test Method Tensile Strength MD Kpsi17.0 to 35.0 ASTM D882 (machine direction) Tensile Strength TD Kpsi 20.0to 40.0 ASTM D882 (transverse direction) Elongation at Break MD % 150.0to 350.0 ASTM D882 Elongation at Break TD % 100.0 to 300.0 ASTM D882Light Transmittance % 87.0 to 94.0 ASTM D1003 Haze % 0.20 to 1.5  ASTMD1003 Gloss (in) 165 to 205 ASTM D523 Gloss (out) 165 to 205 ASTM D523Friction Coefficient, s 0.30 to 0.75 ASTM D1894 Friction Coefficient, k0.25 to 0.70 ASTM D1894 Surface Tension (in) Dyne/cm 30 to 40 ASTM D2578Surface Tension (out) Dyne/cm 30 to 40 ASTM D2578 Color L 94 to 98 ASTME1347 Color a −0.3 to 0.1  ASTM E1347 Color b −0.40 to 0.8  ASTM E1347UV Transmittance (370 nm) % 0 to 4 ISO 9050

When it is to be used as a standalone face shield (i.e. without beingmounted to a surface 10), the protective lens stack 100 may have thefollowing specifications. The PET film 112 of the base layer 110 mayhave a thickness of 4 mil to 20 mil, preferably 7 to 8 mil (e.g. 7.5mil). The PET film 122 of each of the one or more removable lens layers120 may have a thickness of 1 to 4 mil (e.g. 2 mil). In one exampleconstruction, two removable lens layers 120, each comprising a 2 mil PETfilm 122 and an adhesive 124, may be stacked on top of a 7.5 mil indexmatched UV blocking PET film 112 serving as the base layer 110. In theresulting protective lens 100, haze may be 1% and visible lighttransmission (VLT) may be 90%. The base layer 110 and the two removablelens layers 120 may each have less than 0.5% haze.

When the protective lens stack 100 is to be surface mountable (e.g. tothe surface 10 shown in FIG. 1), the base layer 110 may further includean adhesive 114 disposed on a bottom surface of the PET film 112. Theadhesive 114 may have a pull strength greater than that of the adhesive124 of the removable lens layer(s) 120. As such, the wearer of theprotective lens stack 100 can easily remove one of the removable lenslayer(s) 120 without accidentally removing the entire protective lensstack 100 from the surface 10. Examples of such an adhesive 114 may befound in U.S. Pat. Nos. 9,295,297 and 6,536,045, incorporated byreference above. The adhesive 114 may be produced or selected such thatthe refractive indices of the base layer 110 and the one or moreremovable lens layers 120 remain matched to within 0.2.

As noted above, the surface 10 may be an existing window of a surgicalhelmet, hood, or gown. It is also contemplated that the surface 10 maymore generally be any kind of window. In particular, the surface 10 thatthe protective lens stack 100 is affixed to may be a windshield of a caror other vehicle, such that the removable lens layers 120 may be peeledoff as the windshield becomes soiled with mud and other debris. In thisregard, the protective lens stack 100 may be used together with thesystems and methods described in Applicant's U.S. Patent ApplicationPub. No. 2020/0247102, entitled “Thermoform Windshield Stack withIntegrated Formable Mold,” and U.S. patent application Ser. No.16/866,392, filed May 4, 2020 and entitled “Protective Barrier forSafety Glazing,” the entire contents of each of which is expresslyincorporated by reference herein. In its surface mountable form as usedon a windshield, the protective lens stack 100 may have the followingspecifications: The PET film 112 of the base layer 110 may have athickness of 2 mil to 7 mil, preferably 3 to 4 mil. The PET film 122 ofeach of the one or more removable lens layers 120 may have a thicknessof 2 to 7 mil (e.g. 3 to 4 mil). In the resulting protective lens 100,haze may be 2% and visible light transmission (VLT) may be 70%.

In the illustrated example, a 3-ply protective lens stack 100 is shown,having two removable lens layers 120 in addition to the base layer 110.However, it should be noted that the disclosure is not intended to belimited in this regard and that the inclusion of additional removablelens layers 120, or alternatively a construction having only a singleremovable lens layer 120, is also contemplated.

The above description is given by way of example, and not limitation.Given the above disclosure, one skilled in the art could devisevariations that are within the scope and spirit of the inventiondisclosed herein. Further, the various features of the embodimentsdisclosed herein can be used alone, or in varying combinations with eachother and are not intended to be limited to the specific combinationdescribed herein. Thus, the scope of the claims is not to be limited bythe illustrated embodiments.

What is claimed is:
 1. A protective lens stack comprising: a base layerincluding a polyethylene terephthalate (PET) film containing a UVblocking additive, the UV blocking additive having a refractive indexthat is matched with that of the PET film to within 0.2, the base layerhaving a light transmittance of 87%-94%, a 370 nm transmittance of 4% orless, and a haze of 0.2% to 1.5%; and one or more removable lens layersstacked on top of the base layer, each of the removable lens layershaving a haze of less than 0.5% and including a PET film that does notcontain the UV blocking additive and a first adhesive disposed on abottom surface of the PET film, wherein refractive indices of the baselayer and the one or more removable lens layers are matched to within0.2, a total haze of the protective lens stack is 2% or less, and avisible light transmission (VLT) of the protective lens stack is 87% orgreater.
 2. The protective lens stack of claim 1, wherein transmissionof the protective lens stack at wavelengths of 320-380 nm is less than5%.
 3. The protective lens stack of claim 1, wherein the UV blockingadditive comprises a hindered amine light stabilizer (HALS).
 4. Theprotective lens stack of claim 1, wherein the UV blocking additivecomprises a UV absorber.
 5. The protective lens stack of claim 4,wherein the UV absorber is selected from the group consisting of carbonblack, rutile titanium oxide, hydroxy benzophenone, and benzophenone. 6.The protective lens stack of claim 1, wherein the UV blocking additivecomprises a quencher.
 7. The protective lens stack of claim 1, whereinthe PET film of the base layer has a thickness of 4 mil to 20 mil. 8.The protective lens stack of claim 7, wherein the thickness of the PETfilm of the base layer is 7 to 8 mil.
 9. The protective lens stack ofclaim 8, wherein the thickness of the PET film of the base layer is 7.5mil.
 10. The protective lens stack of claim 1, wherein the PET film ofeach of the one or more removable lens layers has a thickness of 1 to 4mil.
 11. The protective lens stack of claim 10, wherein the thickness ofthe PET film of each of the one or more removable lens layers is 2 mil.12. The protective lens stack of claim 1, wherein the base layer furtherincludes a second adhesive disposed on a bottom surface of the PET filmcontaining the UV blocking additive, the second adhesive having a pullstrength greater than that of the first adhesive.
 13. The protectivelens stack of claim 12, wherein the PET film of the base layer has athickness of 2 mil to 7 mil.
 14. The protective lens stack of claim 13,wherein the thickness of the PET film of the base layer is 3 to 4 mil.15. The protective lens stack of claim 12, wherein the PET film of eachof the one or more removable lens layers has a thickness of 2 to 7 mil.16. The protective lens stack of claim 15, wherein the thickness of thePET film of each of the one or more removable lens layers is 3 to 4 mil.17. The protective lens stack of claim 1, wherein a total haze of theprotective lens stack is 1%.
 18. A single-use face shield to be worn byoperating personnel in a medical operating room comprising: a base layerincluding a polyethylene terephthalate (PET) film having a thickness of4 mil to 20 mil and containing a UV blocking additive to protect theoperating personnel's face and eyes from lighting in the medicaloperating room, the UV blocking additive having a refractive index thatis matched with that of the PET film to within 0.2, the base layerhaving a light transmittance of 87%-94%, a 370 nm transmittance of 4% orless, and a haze of 0.2% to 1.5%; and one or more removable lens layersstacked on top of the base layer, each of the removable lens layershaving a haze of less than 0.5% and including a PET film that does notcontain the UV blocking additive and has a thickness of 1 to 4 mil andan adhesive disposed on a bottom surface of the PET film, whereinrefractive indices of the base layer and the one or more removable lenslayers are matched to within 0.2, a total haze of the face shield is 2%or less, and a visible light transmission (VLT) of the face shield is87% or greater.
 19. A surface mountable protective lens stackcomprising: a base layer including a polyethylene terephthalate (PET)film having a thickness of 2 mil to 7 mil and containing a UV blockingadditive, the UV blocking additive having a refractive index that ismatched with that of the PET film to within 0.2, the base layer having alight transmittance of 87%-94%, a 370 nm transmittance of 4% or less,and a haze of 0.2% to 1.5%; one or more removable lens layers stacked ontop of the base layer, each of the removable lens layers having a hazeof less than 0.5% and including a PET film that does not contain the UVblocking additive and has a thickness of 2 to 7 mil and a first adhesivedisposed on a bottom surface of the PET film, wherein the base layerfurther includes a second adhesive disposed on a bottom surface of thePET film containing the UV blocking additive, the second adhesive havinga pull strength greater than that of the first adhesive, whereinrefractive indices of the base layer and the one or more removable lenslayers are matched to within 0.2, a total haze of the protective lensstack is 2% or less, and a visible light transmission (VLT) of theprotective lens stack is 87% or greater.
 20. A protective lens stackcomprising: a base layer including a polyethylene terephthalate (PET)film containing a UV blocking additive, the base layer having less than0.5% haze, a light transmittance of 87%-94%, and a 370 nm transmittanceof 4% or less, the UV blocking additive having a refractive index thatis matched with that of the PET film to within 0.2; and one or moreremovable lens layers stacked on top of the base layer, each of theremovable lens layers having less than 0.5% haze and including a PETfilm that does not contain the UV blocking additive and a first adhesivedisposed on a bottom surface of the PET film, wherein a total haze ofthe protective lens stack is 2% or less and a visible light transmission(VLT) of the protective lens stack is 87% or greater.